The Ah-receptor is a soluble protein which mediates an individuals response to a variety of drugs, carcinogens and toxic agents. Chemicals which interact with the Ah-receptor, include a variety of environmental contaminants (dioxins, PCBs, PBBs, benzo(a)pyrene and a variety of natural products (flavones, carbazoles etc). One of the most potent agonists of the Ah-receptor is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or "dioxin"). TCDD is the prototype for a large family of highly toxic carcinogenic and teratogenic environmental contaminants. Poland A., Knutson, J. C., Ann. Rev. Pharmacol. Toxicol. 22:517-554 (1982). Members of this family include a number of halogenated dibenzo-p-dioxin, dibenzofuran, and biphenyl isomers which induce a variety of receptor-mediated toxic responses, including a severe wasting syndrome, epidermal hyperplasia and metaplasis, tumor promotion and thymic involution.
The Ah receptor is believed to reside primarily in the cytosol. While in the cytosol, the Ah receptor is associated with a dimer of the 90 kDa heat shock protein (hsp 90). It is believed that hsp 90 holds the Ah receptor in a conformation capable of binding ligand, but unable to bind to DNA. Upon binding of a ligand, the Ah receptor undergoes a temperature dependent activation, dissociates from the hsp 90, translocates from the cytosol to the nucleus, displays an increased affinity for specific DNA enhancer elements, known as the dioxin responsive elements (DRE) found in the nucleus. Enhancer elements increase transcriptional efficiency, often independent of their orientation and distance with respect to the promoter.
Once translocated to the nucleus, the Ah receptor dimerizes with the Ah receptor nuclear translocator (ARNT) protein. The Ah receptor-ARNT complex exhibits enhanced affinity for the DREs. The binding of the Ah receptor-ARNT complex to the DRE initiates transcription of the mRNA for the CYP1A1 gene. See Durrin, L. K., Jones, P., B. C., Fisher, J. M. Galeazzi, D. R., and Whitlock, J. P., Jr., J. of Cell. Biochem. 35:153-160 (1987); citing Adesnick, M., Atchison, M., Crit. Rev. Biochem. 19:247-305 (1985) and Lu Ayh, Wet SB., Pharmacol. Rev. 31:277-295 (1979). The CYP1A1 gene encodes an isozyme of the cytochrome P450 enzyme family. Cytochrome P450 enzymes catalyze the oxygenation of many endogenous and exogenous lipophilic substrates and are involved in a variety of metabolic activities.
The photoaffinity ligand, [.sup.125 I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, covalently labels the Ah-receptor from a number of species, tissues and cell types. Poland, A., Glover, E., Ebetino, F. H. & Kende, A. S., J. Biol. Chem. 261:6352-6365 (1986). These photoaffinity labeling studies demonstrated that the Ah-receptor exhibits significant polymorphism, both between species and within different strains of the same species. For example, four different allelic forms of the Ah-receptor have been identified in inbred strains of mice: Ah.sup.b-1 allele (C57 strains)=95 kD, Ah.sup.b-2 allele (e.g., C3H strain)=104 kD, Ah.sup.b-3 allele (Mus spretus)=105 kD, and Ah.sup.d allele (e.g., DBA strain)=104 kD. The Ah.sup.d allele encodes a receptor with a 10-100-fold lower affinity for agonist than the Ah.sup.b-1 or Ah.sup.b-2 alleles. Poland, A. & Glover, E., Mol. Pharm. 11:389-398 (1975); Okey, A. B., Vella, L. M. & Harper, P. A., Mol. Pharm. 35:823-830 (1989); Poland, A., Palen, D., Glover, E., Mol. Pharm. 46:915-921 (1994).
The purification of the Ah-receptor from C57BL/6J mouse liver has been described. Bradfield, C. A., Glover, E. & Poland, A., Mol. Pharm. 39:13-9 (1991). To confirm the identity of this purified protein, its N-terminal amino acids has been sequenced and the corresponding peptide synthesized. Poland, A., Glover, E. & Bradfield, C. A., Mol. Pharmacol. 39:20-6 (1991).